1. Field of the Invention
This invention relates generally to a method for reactivating a catalyst comprising a Group VIII noble metal and more particularly concerns a method for reactivating the aforesaid catalyst for use in the purification by hydrogenation of crude terephthalic acid produced by the liquid phase oxidation of p-xylene in the presence of a catalyst comprising cobalt, manganese and bromine components.
2. Discussion of the Prior Art
Polymer grade or "purified" terephthalic acid is the starting material for polyethylene terephthalate, which is the principal polymer for polyester fibers, polyester films, and resins for bottles and the like containers. Purified terephthalic acid is derived from relatively less pure, technical grade or "crude" terephthalic acid by purification of the latter utilizing hydrogen and a noble metal catalyst as described in Meyer, U.S. Pat. No. 3,584,039.
Impurities present in crude terephthalic acid include partially-oxidized products such as p-toluic acid and 4-carboxybenzaldehyde. These impurities are usually present in the crude terephthalic acid in relatively significant amounts. In addition, color-forming precursors and color bodies that are believed to be of the benzil, fluorenone and/or anthraquinone type are present as impurities. While p-toluic acid is an impurity that can be readily removed from crude terephthalic acid solutions by wellknown cooling-and-crystallization techniques, other impurities such as 4-carboxybenzaldehyde and the aforesaid color forming precusors and color bodies are more difficult to remove from crude terephthalic acid solutions.
One approach to purification of crude terephthalic acid involves first converting the 4-carboxybenzaldehyde to a more soluble product that is more readily separable upon crystallization of the terephthalic acid. To that end, aqueous solutions of crude terephthalic acid are hydrogenated in the presence of a noble-metal catalyst, such as palladium on an active carbon support. This hydrogenation step also converts the various color forming precusors and color bodies present in the crude terephtahlic acid to colorless products. Another related purification-by-hydrogenation process for aromatic polycarboxylic acid produced by the liquid phase catalytic oxidation of polyalkyl aromatic hydrocarbons is described in Stech et al., U.S. Pat. No. 4,405,809. Other such purification-by-hydrogenation processes have suggested using a combination noble metal catalyst e.g., a palladium/rhodium (Pd/Rh) catalyst on a porous carbonaceous support for purification of crude aqueous terephthalic acid solutions. (See, e.g., Puskas et al., U.S. Pat. Nos. 4,394,299 and 4,467,110.)
However, for reasons that have not generally been fully understood, a marked decrease with time has been observed in the activity of the aforesaid noble metal catalyst employed in the aforesaid purification step. The occurrence of this catalytic deactivation is highly undesirable from the standpoint of large commercial operations. In particular, such decrease in catalyst activity is disadvantageous in continuous or semi-continuous operations wherein the catalyst is used for relatively long periods of time. Eventually such catalysts must be taken out of service and replaced by fresh catalysts as product specifications for purified terephthalic acid are exceeded. Therefore, it is essential to develop a method for reactivating the aforesaid noble metal catalyst for use in the aforesaid purification by hydrogenation of crude terephthalic acid.
Methods have been disclosed for reactivating a catalyst comprising a Group VIII metal by washing the catalyst with an alkaline solution. For example, Miller, U.S. Pat. No. 3,650,983, discloses that a palladium catalyst which is employed in the synthesis of vinyl acetate from ethylene, acetic acid and oxygen experiences a serious decrease in catalytic activity during the aforesaid synthesis of vinyl acetate and that the activity of this catalyst for the same synthesis can be regenerated by washing the catalyst with an alkaline solution. Miller discloses furthermore that neither the amount of the alkaline solution employed per given weight of the catalyst nor the concentration of alkali metal or alkaline earth metal salt or hydroxide in the alkaline solution is critical, and each can vary over a wide range. For example, the concentration of the alkali metal or alkaline earth metal salt or hydroxide in the alkaline solution can be 0.25 to 30 percent by weight, and the amount of alkaline solution employed can be 0.1 to 10 liters per 350 grams of catalyst. The regeneration is carried out at ambient temperature and pressure.
Yamauchi et al., U.S. Pat. Nos. 4,147,660; 4,190,554 (a continuation-in-part of U.S. Pat. No. 4,147,660) and 4,228,033 (a divisional of U.S. Pat. No. 4,190,554) disclose that a platinum group catalyst which is employed in the catalytic reaction of hydrocarbons (optionally containing an oxygen atom) suffers a loss of catalytic activity during the course of the catalytic reaction and that the activity of such catalyst for the same reaction can be regenerated by contact with at least one agent selected from an inorganic alkaline substance and a reducing substance in an aqueous medium. The inorganic alkaline substance may be an inorganic alkaline compound containing at least one alkali metal or alkaline earth metal. The concentration of the inorganic alkaline substance in the aqueous medium may be varied depending upon the amount of the catalyst component deposited on the carrier material, the degree of lowered catalytic activity, the temperature and pressure at which the reactivation treatment is carried out and the like. In general, however, the concentration of the inorganic alkalene substance in the aqueous medium is usually from about 0.001 to 10N, preferably from about 0.001 to 5N. The temperature at which the treatment with the inorganic alkaline substance is performed is generally from about 5.degree. C. to 250.degree. C. and can be varied widely depending on the extent of lowered catalytic activity, the composition of the catalyst, and the kind of the inorganic alkaline substance employed. The pressure is typically selected so as to maintain the aqueous system for the aforesaid treatment in a liquid state.
It is also known in the art to employ a multi-step wash of the aforesaid noble metal catalyst first with hot water at 250.degree. C.-300.degree. C., second with cooler water at 20.degree. C.-100.degree. C. and third with a dilute solution of from about 4 to about 8 weight percent of sodium hydroxide in order to reactivate the aforesaid catalyst for use in the purification by hydrogenation of crude terephthalic acid. However, those of ordinary skill in the art to which this invention pertains have avoided the use of more concentrated solutions of alkali or alkaline earth metals on the belief that such solutions would be deleterious to the efficacy of the noble metal catalyst in the purification by hydrogenation of crude terephthalic acid. Furthermore, there is no known prior use or disclosure of a sequence of wash steps involving a wash with a more concentrated solution of from about 12 to about 30 weight percent of ammonium or alkali or alkaline earth metal ions.
It has now been found that the stability of commercially available noble metal-on-carbon catalysts in the aforesaid purification by hydrogenation of crude terephthalic acid (produced by the liquid phase oxidation of p-xylene in the presence of a catalyst comprising colbalt, manganese and bromine components) can be improved and their useful catalytic lifetimes in such purification extended by the practice of the method of the present invention.